Field
This application generally relates to electric machines with coils or windings (e.g., generators, motors), and more particularly to systems, apparatus and methods that configure coils or windings of multistage electric machines.
Description of the Related Art
For conventional fluid-flow electrical-generation turbine systems, such as wind turbine systems, in which the energy source is variable (i.e., the fluid speed and/or the rate of flow of the fluid varies over time), the amount of energy captured from the energy source may only be a fraction of the total of the capturable energy. For example, in a typical wind farm, that fraction may be one half, or less of the total capturable energy.
The power flow though a variable-speed conventional turbine/generator/transformer system is restricted in the range of power that can be output, i.e., from a minimum output power to a rated output power, due to limitations of the generator, the power converter (if present), and/or the output transformer used within the system. This restriction arises because conventional electromagnetic generators have reduced efficiency at lower power levels, as does the power converter (if present) and particularly the transformer that couples power to the electrical load. As a result, for conventional variable-speed turbine/generator/transformer systems an engineering design decision is usually made to limit the power rating of the generator (and any associated power converter, power conditioner or power filter, if present) and the associated output transformer so as to optimize efficiency over a restricted range of power. Therefore, at the extremes of normal-operating fluid speeds, i.e., at a low fluid speed and especially at a high fluid speed, less power is coupled into the turbine than is otherwise possible to extract from the fluid energy source. For a given design of turbine diameter, and possibly axial length, this translates, over time, into less energy capture than the turbine may be capable of transmitting to the generator.
To increase energy capture in situations in which the energy source has a variable speed of fluid driving the turbine, and in which the turbine may have a variable speed of rotation, a multi-stage generator may be used in the turbine system. A multi-stage generator is an electric machine operating as an electrical generator that takes mechanical energy from a prime mover and generates electrical energy, usually in the form of alternating current (AC) power. Such a multi-stage generator is disclosed in U.S. Pat. No. 7,081,696 and U.S. Patent Application Publication No. 2008088200. An advantage of a multi-stage generator over a conventional generator is that a multi-stage generator can be dynamically sized depending on the power output of the turbine. A conventional generator is effective at capturing energy from the energy source over a limited range of fluid speeds, whereas a multi-stage generator is able to capture energy over an extended range of fluid speeds of the energy source, due to staged power characteristics.
The electrical power that is generated from a multi-stage generator is variable in nature, meaning the output power waveforms produced may vary from time to time, for example in: voltage amplitude; current amplitude; phase; and/or frequency. Additionally a multi-stage generator may include a plurality of induction elements, each of which generates its own power waveform, which may differ in voltage amplitude, current amplitude, phase, and/or frequency, from that generated by other induction elements within the generator. An electrical load such as an electric utility power grid may not be capable of directly consuming the electrical power that is generated by a multi-stage generator, as the power generated may not be in a suitable form, for example with respect to waveform shape as a function of time, voltage amplitude, current amplitude, phase, and/or frequency, as may be required by the electrical load. An electrical load such as a utility power grid typically expects from a turbine electrical generation system a single-phase, or split-phase, or 3-phase voltage or current waveform that is usually sinusoidal, and relatively stable. However, a multi-stage generator typically generates varying waveforms.
Past attempts to maintain a consistent output range voltage from such generators have required either the linking together of multiple generators, each optimized for a particular input range, or complex switching networks. These solutions are costly and complex, which in turn, makes them less reliable and limits the available switching range.
U.S. Pat. No. 3,984,750 is directed to an alternator-rectifier unit in which separate three-phase windings are connected to individual rectifiers arranged for series-parallel switching to improve current-voltage characteristics. Notably, such employs switching on the direct current (DC) side of the rectifier. Such circuit is associated with relatively high losses, experiencing four diode voltage drops. Such is not extended beyond a single coil configuration switch.
U.S. Patent Application No. 2007/0182273 describes circuitry for configuring generator coils in various series/parallel combinations. This disclosure has 4 coils configured using 14 switches. Each switch carries multiple coil loads, up to full section current in parallel case. The system uses 12 switches in circuit for a series case, and up to 10 switches in circuit in parallel case.
The concept of independent induction elements and the ability to electrically configure such elements in various configurations offers benefits, however, manufacture of such a design may be challenging and expensive. If the wiring outputs of the machine's induction elements are independently run to a common configuration controller, the numbers of wires could become unmanageable. For example, if the generator had 50 coils per stack (i.e., 50 induction elements) that would result in 100 wires connecting to each stack. For a six stack machine, 600 wires would be used, that must all be properly connected to the appropriate contacts on a single configuration controller. It would be highly challenging to track the wires during assembly, and there are numerous other challenges that result.
New systems, methods and apparatus that allow variable coil configuration of electric machines are desirable.